First Trench Investigation Along The Gargano Fault, Apulia Foreland, Southern Italy
by
Luigi Piccardi
CNR-Istituto di Geoscienze e Georisorse, Sezione di Firenze, Via G. La Pira, 4, Firenze, Italy
Coauthors: L. Ferreli (ANPA, Agenzia Nazionale per la Protezione dell'Ambiente, Via V. Brancati, 48, 00144, Roma, Italy), A.M. Michetti (Dipartimento di Scienze Chimiche, Fisiche e Matematiche, Università dell'Insubria, Via Lucini, 3, 22100, Como, Italy), E. Tondi (Dipartimento di Scienze della Terra, Università di Camerino, Via Gentile III da Varano, 62032 Camerino, Italy), E. Vittori (ANPA, Agenzia Nazionale per la Protezione dell'Ambiente, Via V. Brancati, 48, 00144, Roma, Italy)

The Gargano promontory, a carbonate massif belonging to the Adria microplate, the foreland of both the Apenninic and Dinaric mountain chains, is a well-known seismogenic area. The strongest, well-documented historical earthquake in this area is the 30.07.1627 event, which hit the western part of the Gargano structure with an epicentral intensity of X MCS (estimated magnitude 6.6 - 6.7). Nevertheless, paleoseismological data on the region are still scarce, and no trench fault stratigraphy has been analyzed along the major, E-W trending, right-lateral strike-slip fault that cuts through the whole massif, the South Gargano Fault (SGF). The over 60 km long on-shore trace of the SGF is clearly segmented. The western, ca. 30 km long, fault segment arguably ruptured during the 1627 earthquake. Our investigation focused on the eastern fault segment, the Monte Sant'Angelo Fault (MSF), in order to verify the associated seismic potential. The 20 km long on-shore sector of the MSF shows beautiful evidence for recent surface faulting, such as young bedrock scarps, banded limestone fault planes and deflected streams. Therefore, we selected two trench sites at the base of two prominent fault escarpments, where the MSF is characterized by significant vertical components of fault displacement.

At the first site, near Mattinata, the up to 5 m deep trench exposed faulted slope deposits C14-dated from at least 42000 yr BP to present in the hangingwall of the MSF. Due to local topography and lithology, the excavation could not carve into the carbonatic bedrock footwall.

The second trench, up to 4 m deep, located near Monte Sant'Angelo, exposed colluvial and volcanoclastic deposits C14-dated from 27000 yr BP to present. Here we could excavate across both hangingwall and footwall, which made possible to evaluate the total latest Pleistocene to Holocene vertical displacement.

The preliminary results allow to document that: i) the recent vertical slip-rate of the MSF exceeds 0.2 to 0.3 mm/yr; ii) the MSF moves with incremental slip episodes with vertical surface offset up to several decimeters; iii) based on the evidence for coseismic surface faulting observed in the trenches and assuming a rupture length in the order of 20+ km, the seismic potential of the MSF segment would be similar to that displayed by the western SGF segment during the 1627 event.

Date received: March 28, 2002